1. Field of the Invention
The present invention relates to a semiconductor element formed using a semiconductor film with a crystal structure and a method for manufacturing the same, and relates to a semiconductor device equipped with a circuit which integrated the semiconductor element and a method for manufacturing the same. This invention especially relates to a thin film diode using a Field Effect Transistor (Typically Thin-Film Transistor) and a crystalline semiconductor film. This Field Effect Transistor forms a channel formation region by a crystalline semiconductor film, which is formed on an insulating surface as a semiconductor element.
2. Related Art
Technology of forming semiconductor elements such as a transistor produced in such a way that an amorphous silicon film is formed on an insulating substrate and crystallized, is developed. Especially the technology of irradiating laser beams and crystallizing the amorphous silicon film is applied to the manufacture technology of a thin-film transistor (TFT). A transistor produced using semiconductor films (crystalline semiconductor films) with crystal structure is applied to a plane type display device (flat panel display) represented by a crystal display device.
Technology of re-crystallization of a damage layer and an amorphous semiconductor film formed on a semiconductor substrate or semiconductor films and the technology of crystallization of the amorphous semiconductor films formed on the insulating surface is developed by using laser beams in a semiconductor manufacture process. Widely used laser beams for a laser oscillation device applied for this semiconductor manufacture process is represented by a gas laser such as an excimer laser, and a solid-state laser such as a YAG laser.
An example of crystallization of an amorphous semiconductor film by irradiation of laser beams is disclosed in JP-A-62-104117, which proposes a poly-crystallization of the amorphous semiconductor film by high-speed scan with the scanning speed of laser beams set to more than a diameter of beam spot×5000/second, without making the amorphous semiconductor film result in a perfect melting state. In addition, U.S. Pat. No. 4,330,363 discloses a technology to form a single crystal region substantially on a semiconductor film, which is formed in the shape of an island by irradiating the extended laser beams. Or JP-A-8-195357 discloses a crystallization method of an amorphous semiconductor film using such as a laser processing apparatus, which processes a light beam in linear form before irradiation in an optical system.
Further, JP-A-2001-144027 proposes a technology such that crystalline semiconductor films with large grain size are formed by irradiating laser beams of a second harmonic wave onto the amorphous semiconductor films using solid laser oscillation device such as Nd:YVO4 laser. A transistor is thus constituted.
However, when crystallization is made by irradiating the laser beams onto the amorphous semiconductor film formed on the flat surface, the crystal was made into poly-crystals, producing defects such as a grain boundary which was formed improperly. Therefore, crystal orientations without deviation could not be obtained.
Crystal defects are involved in a grain boundary, resulting in a carrier trap. This may be considered as a causative factor that migration degree of electron or holes falls. Also, the semiconductor films with neither deviation nor crystal defects could not be formed due to a volume contraction of the semiconductor films caused by crystallization, thermal stress applied to a ground material, or lattice mismatching. Accordingly, if special methods such as bond-and-etchback SOI (Silicon on Insulator) are excluded, the crystallized or re-crystallized crystalline semiconductor films formed on an insulating surface, could not obtain an equivalent quality to a MOS transistor which is formed on a single crystal substrate.
The above-described flat panel display device and the like have semiconductor films formed on glass substrates to constitute a transistor. However, it was almost impossible to arrange a transistor so as to obviate a grain boundary formed improperly. That is, the grain boundary or crystal defects involved therein unexpectedly could not be eliminated by controlling the crystallizability of the channel formation regions of a transistor strictly. Consequently, this produced a causative factor of not only the inferior electrical property of a transistor, but also variation in each element characteristics.
Especially when crystalline semiconductor films are formed on a non-alkali glass substrate currently used abundantly industrially by using laser beams, the focus of the laser beams varies in response to the influence of the surge of the non-alkali glass substrate itself, involving a problem of causing crystalline variation as a result. Furthermore, in order for a non-alkali glass substrate to avoid contamination by the alkaline metal, it is necessary to prepare protection films such as insulating films, as ground films. And it was almost impossible to form thereon the crystalline semiconductor films with large grain size and with no grain boundary and crystal defects, which were eliminated.